Organometallics 2010, 29, 3589–3592 DOI: 10.1021/om100507r
3589
Exploring the Scope of Nitrogen Acyclic Carbenes (NACs) in Gold-Catalyzed Reactions Camino Bartolome, Domingo Garcı´ a-Cuadrado, Zoraida Ramiro, and Pablo Espinet* IU CINQUIMA/Quı´mica Inorg anica, Facultad de Ciencias, Universidad de Valladolid, E-47071 Valladolid, Spain Received May 24, 2010
The catalytic activity of the recently reported nitrogen acyclic carbene (NAC) complexes of gold(I) has been investigated and compared with the reported activity of other gold(I) and gold(III) complexes. The complexes studied, [AuCl{C(NEt2)(NHTol-p)}], [AuCl{C(NEt2)(NHXylyl)}], and [Au(NTf2){C(NEt2)(NHXylyl)}], are very active in processes such as the rearrangement of homopropargylsulfoxides, the intramolecular hydroamination of N-allenyl carbamates, the intramolecular hydroalkoxylation of allenes, the hydroarylation of acetylenecarboxylic acid ester, and the benzylation of anisole. Although the NAC ligands have not been optimized for the reactions tested, the yields obtained are usually similar and sometimes better than those reported with other catalysts, showing that the presence of N-H bonds and the wider N-C-N angle in the NAC (as compared to the NHC) complexes are not detrimental for the catalysis. For the hydroarylation reaction (where two competing products can be formed), the NAC complexes allow favoring one over the other. For the benzylation of anisole the selectivity is complementary to that obtained using H[AuCl4] as catalyst, and depending on the substrate, the NAC gold(III) complexes outperform the activity of H[AuCl4]. On average, the reactivity found suggests that the basicity of NACs toward gold(I) is very similar to that of NHCs and higher than that of phosphines.
Introduction In contrast to the scarce attention paid just twenty years ago to gold complexes as catalysts, they are now recognized as very active compounds in many organic transformations.1 Much research in the topic has been carried out on classical complexes with phosphine ligands, but in the last years nitrogen heterocyclic carbene ligands (NHCs) have gained
attention,2 because, as an advantage on phosphines, they are not prone to oxidation. Moreover, recently, Bertrand et al. have successfully used very interesting cationic gold(I) complexes with five-membered cyclic (alkyl)(amino)carbene (CAAC) ligands as catalysts in some organic transformations. 3 To the widely used NHC catalysts and the much less exploited CAACs, we recently added two other carbene types, the so-called hydrogen-bonded heterocyclic complexes (HBHCs)4,5 and the nitrogen acyclic complexes (NACs),6 both shown in Scheme 1. Although gold(I) complexes of the NAC type have been long known,7-9 they had never been applied in catalysis previous to our works showing their high catalytic activity in the skeletal rearrangement and in the alkoxycyclization of 1,6-enynes.5,6,10 As we showed by NMR in our previous works,5,6 depending on the ability of the solvent to break the intramolecular
*To whom correspondence should be addressed. E-mail: espinet@ qi.uva.es. (1) Some recent reviews of gold-catalyzed reactions: (a) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. (b) Jimenez-Nu~nez, E.; Echavarren, A. M. Chem. Commun. 2007, 333–346. (c) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395–403. (d) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (e) Hashmi, A. S. K. Gold Bull. 2004, 37, 1–2. (f) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239–3265. (g) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325. (h) JimenezN u~nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326–3350. (i) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351–3378. (j) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395–3442. (k) Shen, H. C. Tetrahedron 2008, 64, 3885–3903. (l) Widenhoefer, R. A. Chem.;Eur. J. 2008, 14, 5382–5391. (m) Krause, N.; Belting, V.; Deutsch, C.; Erdsack, J.; Fan, H. T.; Gockel, B.; Hoffmann-Roder, A.; Morita, N.; Volz, F. Pure Appl. Chem. 2008, 80, 1063–1069. (2) Dı´ ez-Gonz alez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676. (3) (a) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5224–5228. (b) Zeng, X.; Frey, G. D.; Kousar, S.; Bertrand, G. Chem.;Eur. J. 2009, 15, 3056–3060. (c) Lavallo, V.; Frey, G. D.; Kousar, S.; Donnadieu, B.; Bertrand, G. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 13569–13573. (d) Zeng, X.; Frey, G. D.; Kinjo, R.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2009, 131, 8690– 8696. (e) Xiaoming Zeng, X.; Soleilhavoup, M.; Bertrand, G. Org. Lett. 2009, 11, 3166–3169. (f) Zeng, X.; Kinjo, R.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 942–945.
(4) Bartolome, C.; Carrasco-Rando, M.; Coco, S.; Cordovilla, C.; Martı´ n-Alvarez, J. M.; Espinet, P. Inorg. Chem. 2008, 47, 1616–1624. (5) Bartolome, C.; Ramiro, Z.; Perez-Galan, P.; Bour, C.; Raducan, M.; Echavarren, A. M.; Espinet, P. Inorg. Chem. 2008, 47, 11391–11397. (6) Bartolome, C.; Ramiro, Z.; Garcı´ a-Cuadrado, D.; Perez-Galan, P.; Raducan, M.; Bour, C.; Echavarren, A. M.; Espinet, P. Organometallics 2010, 29, 951–956. (7) Bonati, F; Minghetti, G. J. Organomet. Chem. 1973, 59, 403–410. (8) Parks, J. E.; Balch, A. L. J. Organomet. Chem. 1974, 71, 453–463. (9) Minghetti, G.; Bonati, F. Inorg. Chem. 1974, 13, 1600–1602. (10) With the present manuscript finished, Hashmi et al. have published following our previous papers the use of NAC gold carbenes in phenol synthesis and in the hydration of alkynes: Hashmi., A. S. K.; Hengst, T.; Lothsch€ utz, C.; Rominger, F. Adv. Synth. Catal. 2010, 8, 1307–1314.
r 2010 American Chemical Society
Published on Web 07/22/2010
pubs.acs.org/Organometallics
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Organometallics, Vol. 29, No. 16, 2010
Bartolom e et al.
Scheme 1. Types of Gold(I) Carbene Complexes
Table 1. NAC Gold-Catalyzed Rearrangement of Homopropargylsulfoxide
entry
Scheme 2. Types of Carbene Gold(I) Complexes
hydrogen bond in HBHCs, these are structurally similar (cyclic) to the NHC carbenes (in acetone or CH2Cl2) or become similar to the NACs in solvents where the intramolecular hydrogen bond has been split (in MeOH). The fact that the HBHC gold complexes were similarly active in methoxycyclization of enynes, where their cyclic structure is broken,5 and in cyclization reactions in CH2Cl2, where its cyclic structure is preserved, suggested that, except for the possible influence in the spatial arrangement of the substituents, the cyclic or acyclic structure of the carbene is not a crucial difference. In fact both types of gold complexes (NACs and HBHCs) showed similar catalytic activity.5,6 The HBHC ligands require the cumbersome low-yield synthesis of 2-pyridyl isocyanide, but the easy synthesis of NAC metal complexes by simple nucleophilic attack of amines to gold(I) isocyanide complexes (even from commercial isocyanides and amines) (eq 1)11 makes NAC advantageous over NHC complexes to produce a series of gold catalysts and easily tune their electronic and steric characteristics.
However, these NAC complexes (also the HBHCs) have two peculiar features that make them different from the NHCs: (i) In the absence of structural distortions by the substituents, the N-C-N angle at the carbene atom should be about 60°, compared to about 72° for the NHCs, which means that, for similar electronic influences by the substituents, the expected order of carbene basicity should be NACs < NHCs.12 (ii) Due to the method of synthesis, the coordinated carbenes have at least one (for secondary amines) or two (for primary amines) active N-H hydrogen atoms, which might interfere for some applications. These two features could be detrimental for the activity of NACs catalysts, so we decided to examine their scope of application by examining their performance, compared to the best results found in several reported gold-catalyzed processes. (11) For other metal complexes with NACs, see: (a) Michelin, R. A.; Pombeiro, A. J. L.; Guedes da Silva, M. F. C. Coord. Chem. Rev. 2001, 218, 75–112. (b) Vignolle, J.; Cattoe, X.; Bourissou, D. Chem. Rev. 2009, 109, 3333–3384. (12) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717–8724.
c
[Au]
AgX
yield [%]
AgSbF6 AgNTf2 AgNTf2 AgSbF6
62a 75a 82a 76a 60a 30a (34b) 30a 25b 76a (94b) 77a 93
1 2 3 4 5 6
1 1 2 2 3 [AuCl(PPh3)]
AgSbF6
7 8 9
[AuCl(PPh3)] [AuCl(P(p-CF3C6H4)3)] [AuCl(IMes)]
AgNTf2 AgSbF6 AgSbF6
10 11
[AuCl(IMes)] [AuCl2(N-O)]c
AgNTf2
a Our result, 1H NMR yield. b Literature, isolated yield, see ref 16. See ref 15.
This should give us information about the feasibility of application of the NAC-type carbenes in gold catalysis and about the actual nucleophilicity of this structural type of carbenes compared to other ligands used in gold(I) catalysis.
Results and Discussion Complexes 1 and 2 have been reported before6 and were chosen for the tests because they had been found particularly efficient and highly selective in the skeletal rearrangement and methoxycyclization of 1,6-enynes (Scheme 2).6 Complex 3, with the weakly coordinated counteranion bis(trifluoromethanesulfonyl)imidate, was synthesized by reaction of the neutral gold(I) carbene 2 with AgNTf2,13 affording in good yield a white and fairly stable solid. The catalytic reactions chosen to test the activity of the NAC gold(I) complexes were carried out under the standard conditions used for the reported references (this means that the performance with NACs was not optimized) and, except for [Au(NTf2){C(NEt2)(NHXylyl)}] (3), adding a silver salt to extract the chloride in situ to allow for the coordination of the substrate to the Au center.14 1. Rearrangement of Homopropargylsulfoxides. Zhang and Li have demonstrated that dichloro(pyridine-2-carboxylato)gold(III) is active for this reaction in the absence of a silver salt (Table 1, entry 11).15 With gold(I), Saphiro and Toste have reported that phosphine complexes [AuClL] (L = PPh3, P(p-CF3C6H4)3) are active catalysts for the rearrangement of the homopropargylsulfoxide 4 to 1-benzothiepin-4-one 5 in the presence of AgSbF6.16 However, the yield obtained with PPh3 was low (34%, Table 1, entry 6b), and it was even worse with the poorer donor phosphine P( p-CF3C6H4)3 (25%, Table 1, entry 8).16 In our hands (entries 6a and 7), similar poor results have been obtained by using [AuCl(PPh3)] and AgNTf2 instead of AgSbF6 (Table 1, entry 7). (13) Ricard, L.; Gagosz, F. Organometallics 2007, 26, 4704–4707. (14) The reactions in the literature have been repeated in our lab, as suggested by one reviewer, for better comparison. When there is a significant difference, both results are given in the tables. (15) Zhang, L.; Li, G. Angew. Chem., Int. Ed. 2007, 46, 5156–5159. (16) Shapiro, N.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 4160– 4161.
Article
Organometallics, Vol. 29, No. 16, 2010
Table 2. NAC Gold-Catalyzed Intramolecular exo-Hydroamination of N-Allenyl Carbamate
entry
[Au]
AgX
yield [%]
1 2 3
2 3 [AuClP(t-Bu)2(o-biphenyl)]
AgOTs
92a 78a 95b
a
AgOTf
Our result, isolated yield. b Literature, ref 18.
The better donor NHC ligand IMes was reported by Toste to afford 94% yield (Table 1, entry 9b);16 in the same reaction conditions, our yield calculated by 1H NMR was not as high, whether with AgSbF6 or with AgNTf2 as chloride scavenger (entries 9a and 10). The results obtained with 1-3, shown in Table 1, are in the range of activity of their analogous NHC (somewhat better or worse depending on the experiment reference (our own or the data in the literature), which is consistent with the presumption that NACs should be in the order of basicity of NHCs, maybe a bit less basic than them. Under the same conditions NAC complexes compare favorably with phosphines, as expected.17 Thus, the yields in this reaction seem to follow the order of basicity of the ligand. 2. Intramolecular Hydroamination of N-Allenyl Carbamates. As nitrogen and oxygen heterocycles are a part of the structure of a wide range of biologically active systems, the design of new catalysts for the synthesis of heterocyclic compounds is a very attractive field. The gold(I)-catalyzed intramolecular hydroamination of N-allenyl carbamates such as 6 (Table 2) has been reported with an equimolecular mixture of [AuClP(t-Bu)2(o-biphenyl)] and AgOTf (Table 2, entry 3) and is very selective toward exo-hydroamination.18 We find that the NAC gold(I) complexes 2 and 3 are also very active catalysts for this exo-hydroamination. When a mixture of 2 and AgOTs was used, the hydroamination of the allenyl carbamate 6 led to isolation of 2-vinylpyrrolidine 7 in 92% yield (Table 2, entry 1), similar to the yield reported using [AuClP(t-Bu)2(o-biphenyl)] as precatalyst and AgOTf. The reaction carried out using 3 as catalyst with NTf2 as ligand was about as effective (Table 2, entry 2). 3. Intramolecular exo-Hydroalkoxylation of Allenes. NAC gold(I) complexes are also active catalysts in the intramolecular hydroalkoxylation of 2,2-diphenyl-4,5-hexadienol 8. Widenhoefer et al. reported that the regioselectivity of this reaction has a very strong dependence on the counterion.18 For instance, the reaction carried out using a mixture of [AuClP(t-Bu)2(o-biphenyl)] and AgOTf had a very low regioselectivity (Table 3, entry 3), producing a 1.3:1 mixture of tetrahydrofuran 9 and dihydropyran 10 in 85% yield; however, the same reaction using AgOTs instead of AgOTf to extract the chloride was highly regioselective, again in favor of the exo-hydroalkoxylation product 9 (Table 3, entry 4). In the presence of AgOTs, the NAC complex 2 was not only very efficient for the hydroalkoxylation but also highly selective toward 9 (Table 3, entry 1). The reaction using 3, with the labile ligand NTf2, afforded lower yields, (17) Phosphines have been ranked as less basic than NHCs in ref 12. (18) Zhang, Z.; Liu, C.; Kinder, R. E.; Han, X.; Qian, H.; Widenhoefer, R. A. J. Am. Chem. Soc. 2006, 128, 9066–9073.
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Table 3. NAC Gold-Catalyzed Intramolecular Hydroalkoxylation of 2,2-Diphenyl-4,5-hexadienol
entry
[Au]
AgX
Yield [%]
1 2 3 4
2 3 [AuClP(t-Bu)2(o-biphenyl)] [AuClP(t-Bu)2(o-biphenyl)]
AgOTs
94/
